An electric power steering apparatus, in which a steering apparatus of an automobile or a vehicle is energized by an assist load using rotary force of a motor, driving force of the motor is applied to a steering shaft or a rack shaft via reduction gears or by a transmitting mechanism such as gears or a belt so that the steering shaft or the rack shaft is energized by the assist load. Since such conventional electric power steering apparatus generates an assist torque (steering assist torque) accurately, it controls the feedback of motor currents. The feedback control adjusts a motor applied voltage so that a difference between a current command value and a motor current detected value becomes small or zero. The adjustment of the motor applied voltage is made generally by a duty ratio of PWM (pulse width modulation) control.
Japanese Patent Application Laid-Open No. 2003-189658 A (Patent Document 1) discloses a technique such that a motor present position of an electric power steering apparatus is made to respond at a high speed. A control device in the above Patent Document 1 has a command generator 4 which provides a command θref, a load machine 1 and a motor driving device 2 which drives the load machine 1 based on the torque command Tref. The control device is a delay-compensating motor control device which provides a torque command Tref based on a motor present position θm of the motor driving device 2. This control device is provided with a monitoring device 3 which monitors a state of the motor driving device 2 and provides a motor delay position θn which delays from the motor present position θm, a delay compensating observer 6 which provides an estimated motor present position hθm based on the motor delay position θn and the torque command Tref, and a first control device 5 which provides the torque command Tref based on the command θref and the estimated motor present position hθm.
The electric power steering apparatus in the Patent Document 1 is provided with the delay observer which inputs the delay position θn and the torque command Tref therein and outputs the estimated present position so that the motor present position θm is estimated by the estimated motor present position hθm.
In the device of the Patent Document 1, however, an estimating arithmetic expression is complicated, and it is difficult to determine an inertia moment Jm in an electric power steering. In the electric power steering, a lot of parts such as a motor, reduction gears, an intermediate joint, a rack pinion, tires and so on are subjects for driving. Since respective connecting portions have looseness, it is difficult to obtain the inertia moment Jm.
The present applicant of this invention proposes a delay compensation of a back EMF, but it is supposed that a delay between actual motor information (position, angular velocity, voltage, electric current, back EMF and the like) and an information which is used for the control contributes heavily to occurrence of torque ripples (Japanese Patent Application No. 2003-163446). That is to say, when the delay of the information is compensated, the torque ripples are reduced.
The present invention is devised from the viewpoint of the above circumstances and its object is to provide a control device for an electric power steering apparatus in which torque ripples at the time of rotation of a motor are reduced by a simple arithmetic so that smooth and safe assist control can be made with a steering feeling being improved.